This invention pertains to the art of pressure swing adsorption, and more particularly to the art of pressure swing adsorption to purify oxygen. The invention is predominately applicable to the art of splitting oxygen from argon in connection with pressure swing adsorption methods using an adsorbent selective to argon, and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications and uses, such as in connection with removing argon from other gases including air, and may be advantageously employed in various environments. These other applications and uses are considered to be included within the scope of this invention.
Pressure swing adsorption (PSA) is a well-known method for separating gases. Typically, an adsorption column contains an adsorbent material that is selective toward one of the gases that is going to be separated. In standard operation of a pressure swing adsorption column, four steps are followed. First, feed gas (which is to be separated) is delivered under pressure to a previously pressurized column at a feed end thereof. During this step, the gas to be separated (the secondary product or "heavy" gas) is adsorbed onto the selective adsorbent material, and the remaining gas (the primary product or "light" gas) escapes through the product end of the column. As the feed passes through the column, the more strongly adsorbed component ("heavy") is selectively taken up. At the other end of the column, a continuous stream of purified, less strongly adsorbed component is taken off. As the bed reaches capacity, the product stream is closed off, and the blowdown step occurs.
The second or countercurrent blowdown step occurs whereby the column is vented to decrease the pressure. The previously adsorbed secondary product gas readily escapes from the column into a lower pressure area.
Third, the column is subjected to a purge step whereby primary product gas is recycled through the column in a direction countercurrent to the feed step in order to remove any of the adsorbed product remaining in the column and to regenerate the adsorbent bed. The purge step ceases when the purge stream reaches the product end of the column, at which point a valve at the feed end is closed to commence the pressurization step.
During this fourth or pressurization step, purified product continues to flow into the column. The column is pressurized by the purified primary product gas that is admitted through the product end of the column while the feed end is closed. This completes one PSA cycle. The next cycle begins with the feed step described above, and the steps are repeated.
A second PSA column may be operated 180 degrees out of phase with the cycle in the first column. This allows for a continuous feed stream that alternates between columns, as well as a continuous product stream.
With all of the above in mind, it has become desirable to devise a method for using PSA to produce a high purity oxygen product from air. The three major components of air include nitrogen (about 78%), oxygen (about 21%) and argon (about 1%).
It is known in the art of PSA to produce a stream of enriched oxygen from air by using adsorbents such as zeolite 5A or 13X, both of which are selective to the nitrogen component of air. Since argon, like oxygen, is non-polar and is not quadripolar (i.e., has virtually no quadripole moment), it adsorbs in a similar manner to oxygen. As a result of this similarity, most of the argon is present in the product stream with the oxygen when nitrogen selective adsorbents are used. Such stream has an oxygen product purity of roughly 95%, with the remaining 5% being argon.
It has become desirable to develop a method of further purifying the oxygen according to a pressure swing adsorption process. With a feed gas comprising oxygen and argon, the oxygen in such a method would become the primary product, and argon the secondary product. Such method would follow the PSA steps described above, or slight modifications thereto. The adsorbent used in such a method would be selective to argon. The separated oxygen would find usefulness in a variety of environments and applications such as in association with medical, industrial, aeronautical and experimental situations where highly purified oxygen is beneficial.
It is further desirable to develop a method for producing oxygen having at least a 99.6% purity at a recovery rate of at least about 25%. A desirable level of productivity would be about 11Nm.sup.3 /m.sup.3 hr.
The present invention contemplates a method for recovering relatively high-purity oxygen from a gas comprising oxygen and argon at a ratio of about 21:1. The method involves a pressure swing adsorption system using an adsorbent material selective to argon. Favorable operating parameters have been determined as well.